In the manufacture of papers, pigments and other agents are customarily metered into the paper pulp separately rather than mixing them prior to addition to the pulp. This not only eliminated the need for extra mixing equipment and additional work but also permitted far more versatility in the amounts of pigments or other agents added as compared to making up the mixture first. However, certain advantages are available by first preparing mixture of pigments such as a composite pigment of titanium dioxide and calcium carbonate, prior to adding same to the pulp.
Many composite pigments have been described in the art and the majority of these involve TiO.sub.2 as one of the components because of its superior opacifying power. Moreover, because of the relatively high cost of TiO.sub.2, there is considerable interest in forming such composites, or in extending the TiO.sub.2, with relatively low cost materials such as calcium sulfate, calcium carbonate, or sodium aluminum silicates, for example. The present invention is concerned with combinations of TiO.sub.2 and CaCO.sub.3 for use as opacifying agents in paper, and differs from composite pigments described in the prior art in that they are easier and simpler to prepare and in that no chemical reaction is involved in their preparation.
One method of the prior art is described in U.S. Pat. No. 2,170,800 wherein a mixture of TiO.sub.2 and CaSO.sub.4 is reacted with a solution of Na.sub.2 CO.sub.3 to convert the CaSO.sub.4 to CaCO.sub.3 and precipitate a composite pigment comprised of TiO.sub.2 and CaCO.sub.3.
Another method for producing a TiO.sub.2 --CaCO.sub.3 composite pigment is described in U.S. Pat. No. 3,528,838 in which a solution of Na.sub.2 CO.sub.3 and a solution of CaCL.sub.2 are employed. Pigmentary TiO.sub.2 is dispersed in one of these solutions and the two solutions are mixed to precipitate a TiO.sub.2 --CaCO.sub.3 composite pigment.
Still another coalesced composite pigment is described in U.S. Pat. No. 3,832,206 wherein pigmentary TiO.sub.2 is dispersed in a solution of Na.sub.2 CO.sub.3. A slurry of Ca(OH).sub.2 is added to the Na.sub.2 CO.sub.3 --TiO.sub.2 mixture in order to react with the Na.sub.2 CO.sub.3 and precipitate CaCO.sub.3. The CaCO.sub.3 formed in the presence of the TiO.sub.2 pigment particles results in a coalesced composite pigment of TiO.sub.2 --CaCO.sub.3.
In each of the three processes referred to above, the composite pigment product before it can be used in paper making, must be separated from the aqueous phase in which it is formed and washed free of the by-product resulting from the chemical reaction involved in producing CaCO.sub.3. This would be Na.sub.2 SO.sub.4 in case of U.S. Pat. No. 2,170,800, NaCl in the case of U.S. Pat. No. 3,528,838, and NaOH in the case of U.S. Pat. No. 3,832,206.
In the present invention, since no chemical reaction is involved, there is no by-product which must be separated from the TiO.sub.2 --CaCO.sub.3 combination by washing or otherwise. This results in important simplifications of the process, the elimination of costly equipment, such as a vacuum filter, and the elimination of waste water disposal problems. It also eliminates the risk of unreliable performance caused by the greater likelihood of impurities in the composite pigment.
In order to demonstrate the process of this invention, it is necessary to utilize a quantitative means for expressing and comparing the opacifying power of paper fillers. This can be done accurately and precisely by using a measurement called the "scattering coefficient, s," derived from the theory of light scattering developed by Kubelka and Munk and published in 1931, Z. Tech. Physik 12:539 (1931). Application of the Kubelka-Munk Theory to papers did not come until much after 1931, but is now the common means used by the paper industry to evaluate the opacifying power of fillers. There are a number of excellent discussions of the use of scattering coefficient measurements in paper applications such as W. J. Hillend, Tappi, 49:41A (July 1966) and G. A. Hemstock, Tappi, 45:158A (February, 1962).
Table 1 below gives the scattering coefficients, s, for precipitated CaCO.sub.3, and the two forms of TiO.sub.2 in paper. The higher the value of s, the greater the opacifying power of the filler or pigment. It is well recognized that rutile TiO.sub.2 has even more opacifying power than anatase, but it is also more expensive and is used but little in papers. A range in values is given because there are a number of different commercial products of each filler type and these can have different scattering coefficients depending upon particle size distribution, surface treatments on the particles, etc. Also, for any given product, different values for the scattering coefficient in paper can be obtained as a result of experimental errors or differences in the modes of incorporating the fillers in paper. However, a skilled technician following a set procedure can obtain replicate values with a given product which agree within 100 to 200 units.
TABLE 1 ______________________________________ Scattering Coefficients In Paper ______________________________________ s Filler cm.sup.2 /g Identification ______________________________________ ppt. CaCO.sub.3 2000-2400 Purecal-O, BASF Wyandotte or Albaglos, Pfizer, Inc. anatase TiO.sub.2 4000-4700 Titanox AWD 1010, N. L. Industries, Inc. or A-410, New Jersey Zinc Co. rutile TiO.sub.2 5500-7500 Titanox RA 42, N. L. Industries, Inc. ______________________________________